1. Technical Field
The present invention relates generally to a steering control system for a vehicle. More particularly, the invention relates to an improved rear wheel steering control system suitable for a vehicle in which a traction control system (TCS) is incorporated.
2. Background Art
A Japanese Patent First Publication No. 62-149545 discloses a traction control system for a vehicle. This system is adapted for adjusting engine output or braking to control driving torque so that over torque of driven wheels is reduced to provide rotational speeds of the driven wheels greater than that of compliance wheels by a certain percent in response to over a gripping force thereof, i.e., rotational speeds of the driven wheels greater than those of the compliance wheels, thereby effectively improving an initial acceleration characteristics.
A Japanese Patent First Publication No. 62-71761 discloses a four-wheel steering (4WS) system for a vehicle. This system determines a rear wheel steering angle in a preselected relation to a vehicle speed and a steered angle of front wheels and compensates the rear wheel steering angle based on a difference in rotational speeds between the front and rear wheels to enhance a turning limitation, preventing spin and/or drift-out from occurring.
It will be appreciated that a combination of these two systems provide advantages in initial accelerating ability (TCS) and turning stability (4WS) to significantly improve vehicle performance.
However, in a case of the simple combination of the above technologies, a rear wheel steering angle sometimes varies minutely due to control of the TCS with the result that a driver feels uncomfortable during a steering operation.
For example, the TCS is operable to be responsive to detection of driven wheel slippage to close a sub-throttle valve of an engine so as to reduce engine output while restricting driven wheel torque, preventing driven wheel slippage from occurring. By repeating a series of such controls, wheel traction is recovered. Closing and opening operations of the sub-throttle valve are effected by a preselected cycle. This cycle (i.e., a hunting frequency) is set to 1 to 2 Hz for suitable control response.
The 4WS system is operable to be responsive to generation of a difference in rotational speeds between driven and compliance wheels (i.e., slippage) to compensate a rear wheel steering angle precisely. Since the difference in rotational speeds during operation of the TCS varies minutely, caused by synchronizing with the hunting frequency (1 to 2 Hz) compensation tends to vary minutely. Additionally, as the hunting frequency is sufficiently greater than a frequency of normal driving operation by a driver (the typical driving operation is about 0.3 Hz), undesired vehicle motion occurs, causing uncomfortable steering operation for the driver.